(1) Field of the Invention
The present invention relates to a solid state imaging device including a semiconductor substrate on which a plurality of pixel cells have been formed, and in particular to the structure of a pixel region in a MOS type solid state imaging device.
(2) Description of the Related Art
In recent years, MOS type solid state imaging devices have been used as imaging devices in digital still cameras, etc. Each of the MOS type solid state imaging devices has a pixel region in which a plurality of pixel cells have been arranged two-dimensionally (for example, arranged in an array), and a circuit region for driving the pixel cells in the pixel region (Japanese Patent Application Publication No. 2001-230400, and Japanese Patent Application Publication No. 2006-286848). The following describes the structure of the pixel region in a MOS type solid state imaging device, with reference to FIG. 1.
As shown in FIG. 1, each pixel cell in the pixel region includes one photodiode 81 and four transistors (a transfer transistor 82, an amplification transistor 83, a selection transistor 84, and a reset transistor 85). These transistors 81-85 have been formed on a well region in a semiconductor substrate. Also, in a pixel region, a substrate contact (not shown in FIG. 1) has been arranged between pixel cells 80 that are adjacent to each other. The following describes an arrangement of the substrate contact, with reference to FIG. 2. FIG. 2 shows six pixel cells in a pixel region 91, namely red pixel cell 80r1, green pixel cells 80g1-80g3, and blue pixel cells 80b1-80b2.
As shown in FIG. 2, the blue pixel cell 80b1, the green pixel cell 80g3, and the blue pixel cell 80b2 have been arranged from the top left in the pixel region 91, and the green pixel cell 80g1, the red pixel cell 80r1, and the green pixel cell 80g2 have been arranged from the bottom left in the pixel region 91.
Note that the red pixel cell 80r1 includes a color filter that transmits red visible light (the wavelength being in the range of 575 [nm] to 700 [nm]), each of the green pixel cells 80g1-80g3 includes a color filter that transmits green visible light (the wavelength being in the range of 490 [nm] to 575 [nm]), and each of the blue pixel cells 80b1-80b2 includes blue visible light (the wavelength being in the range of 400 [nm] to 490 [nm]).
As shown in FIG. 2, in a MOS type solid state imaging device according to a conventional technique, substrate contacts 801-808 have been formed between adjacent pixel cells in the pixel region 91. The substrate contacts 801-808 have been formed by evenly spaced from the photodiodes 81r1, 81g1-81g3, and 81b1-81b2 of the pixel cells 80r1, 80g1-80g3, and 80b1-80b2.
As shown in FIG. 2, in the MOS type solid state imaging device, the substrate contacts 801 to 808 have been arranged in the pixel region 91, so as to stabilize a well potential. As a result, the transistors 82-85 in each of the pixel cells 80r1, 80g1-80g3, and 80b1-80b2 are operated at high speed and in a stable manner.
However, the MOS type solid state imaging device having the above-mentioned substrate contacts has a problem in which a large shading appears in the output signal due to progress in reducing the size of image pixels. In particular, shading that appears in the pixel cells 80b1 and 80b2 is larger than shading that appears in the other pixel cells, namely the pixel cells 80r1 and, 80g1-80g3, since the pixel cells 80b1 and 80b2 receive blue visible light having a short wavelength. The following describes a mechanism of how shading occurs in a MOS type solid state imaging device according to the conventional technique, with reference to FIG. 3.
As shown in FIG. 3, an isolation 901, wirings 902 (902a, 902b, and 902c), a color filter 903, and a top lens 904 have been formed in a pixel cell on a semiconductor substrate. Also, a substrate contact 800 has been formed on the semiconductor substrate in a portion corresponding to each side of the photodiode 81.
A reference number 701 in FIG. 3 shows a boundary (hereinafter referred to as “dividing ridge 701”) where electrons generated by photoelectric conversion are absorbed by the photodiode 81. The electrons generated by the photoelectric conversion are likely to be concentrated on the photodiode 81, by repelling the existence of the substrate contact 800. As a result, the dividing ridge 701 spreads toward the substrate contact 800, in a shallow area of the semiconductor substrate.
Here, as shown in Japanese National Publication of the Translated Version of PCT Application, No. 2002-513145, a large part of blue visible light having a short wavelength (400 [nm] to 490 [nm]) is absorbed by the semiconductor substrate (silicon substrate) at a depth of approximately 0.2 [μm] to 0.5 [μm]. Consequently, as shown in FIG. 3, when the blue visible light enters the semiconductor substrate of a MOS type solid state imaging device according to the conventional technique, electrons are generated in a shallow area of the semiconductor substrate by the photoelectric conversion. Therefore, in a MOS type solid state imaging device according to the conventional technique, a sensitivity characteristic varies depending on a relative positional relationship between the substrate contact 800 and the photodiode 81, which particularly have a great impact on the pixel cells 80b1 and 80b2 that receive the incidence of the blue visible light, as described above.
Furthermore, shading occurs by a difference in an incident direction of blue light. In other words, the blue light enters pixels that are positioned upward (hereinafter referred to as “upper pixels”) in the entirety of a pixel array of a solid state imaging device, in a manner that the blue light enters obliquely from a lower direction. Therefore, the photoelectric conversion occurs in an upper portion (in the vicinity of a region where the substrate contacts have been arranged) of the photodiode in each of the upper pixels, resulting in the upper pixels having a high sensitivity. On the contrary, the blue light enters pixels that are positioned downward (hereinafter referred to as “lower pixels”) in the entirety of the pixel array of the solid state imaging device, in a manner that the blue light enters obliquely from an upper direction. Therefore, the photoelectric conversion occurs in a lower portion (in the vicinity of a region having no substrate contact) of the photodiode in each of the lower pixels, resulting in the lower pixels having a low sensitivity. As described above, shading occurs when a sensitivity difference occurs between the upper pixels and the lower pixels in the pixel array, due to a difference in the position of each substrate contact with respect to the direction of incident light.
Also, in a solid state imaging device having a multi-pixel one-cell structure, such as a four-pixel one-cell structure, there are (i) a photodiode for a blue pixel, the photodiode including the substrate contact arranged in the vicinity thereof and (ii) a photodiode for a blue pixel, the photodiode including the substrate contact not arranged in the vicinity thereof. This also causes a sensitivity difference between the photodiodes in the above-mentioned two pixels.